Defence officials said the Syrian planes were leaving as the coalition aircraft arrived, meaning they did not engage in combat, but that the US would not hesitate to defend its forces.
Captain Jeff Davis said the American jets were sent to the north-eastern city of Hasakah on Thursday after two Syrian air force SU-24s started air strikes near where coalition forces were conducting operations on the ground.
“It troubles us when we see regime airstrikes in Hasakah in an area where it's well known by everybody, to include the [Assad] regime, that the coalition is actively engaged in operations against Isis,” the Department of Defence spokesperson said.
“We view instances that place coalition personnel at risk with the utmost seriousness and we do have the inherent right of self-defence when US forces are at risk.”
Elite troops are training and supporting the Kurdish People’s Protection Units (YPG) in the area, where rebel territory provides an eastern base for an expected advance on Isis’ de-facto capital of Raqqa.
But as the Syrian civil war continues, regime forces have turned their fire on Kurdish areas of Hasakah for the past two days, causing thousands of civilians to flee a series of air raids.
Capt Davis said Thursday’s incident was the first time American planes had been scrambled in response to Syrian bombing, and the first known wide-ranging attack against the YPG by the regime.
Coalition military personnel attempted to contact the pilots over the “guard frequency” to prevent the confrontation but received no response, and moved in after being assured by Russian officials that their jets were not responsible for the bombing.
As allies of Assad, the Russians were asked to tell the Syrian government that US aircraft would move in if their troops were threatened, Capt Davies said.
In pictures: Russian air strikes in Syria
In pictures: Russian air strikes in Syria
Syrian boys cry following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo
Russian defense ministry spokesman Major General Igor Konashenkov speaks to the media in Moscow, Russia. Konashenkov strongly warned the United States against striking Syrian government forces and issued a thinly-veiled threat to use Russian air defense assets to protect them
Syrians wait to receive treatment at a hospital following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Alepp
Russian Deputy Defense Minister Anatoly Antonov speaks at a briefing in the Defense Ministry in Moscow, Russia. Antonov said the Russian air strikes in Syria have killed about 35,000 militants, including about 2,700 residents of Russia
Jameel Mustafa Habboush, receives oxygen from civil defence volunteers, known as the white helmets, as they rescue him from under the rubble of a building following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo
Civil defence members rest amidst rubble in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria
A girl carrying a baby inspects damage in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria
Civilians and civil defence members look for survivors at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria
Civilians and civil defence members carry an injured woman on a stretcher at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria
Volunteers from Syria Civil Defence, also known as the White Helmets, help civilians after Russia carried out its first airstrikes in Syria
The aftermath of Russian airstrike in Talbiseh, Syria
Smoke billows from buildings in Talbiseh, in Homs province, western Syria, after airstrikes by Russian warplanes
Russian Air Forces carry out an air strike in the ISIS controlled Al-Raqqah Governorate. Russia's KAB-500s bombs completely destroy the Liwa al-Haqq command unit
Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria
Â© TASS/ITAR-TASS Photo/Corbis
Russia claimed it hit eight Isis targets, including a "terrorist HQ and co-ordination centre" that was completely destroyed
A video grab taken from the footage made available on the Russian Defence Ministry's official website, purporting to show an airstrike in Syria
A release from the Russian defence ministry purportedly showing targets in Syria being hit
Russia launched air strikes in war-torn Syria, its first military engagement outside the former Soviet Union since the occupation of Afghanistan in 1979. Russian warplanes carried out strikes in three Syrian provinces along with regime aircraft as Putin seeks to steal US President Barack Obama's thunder by pushing a rival plan to defeat Isis militants in Syria
Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria, a thousand kilometres away. The targets include ammunition factories, ammunition and fuel depots, command centres, and training camps
Â© TASS/ITAR-TASS Photo/Corbis
The US and Russia have established a special communications channel to avoid mid-air conflict as they continue to back opposing sides of the civil war, but no such contact exists with the Syrian government.
Capt Davis said no US personnel were injured and that airspace would be closely monitored by extra patrols to ensure their safety, adding: “As we've said in the past, the Syrian regime would be well-advised not to interfere with coalition forces or our partners.”
The situation threatens to bring the international coalition into direct conflict with Assad’s forces for the first time since US planes started an anti-Isis bombing campaign in September 2015.
Faysal Itani, a senior fellow at the Atlantic Council think tank, said it was easy for the Syrian government to avoid bombing large bases but far more risky to strike areas close to YPG forces being trained by the coalition.
“If the regime continues bombing in close proximity to US forces, then simply I think the United States will shoot down the regime aircraft,” he added.
Britain, France and several other nations are also conducting air strikes, which are predominantly targeting the so-called Islamic State and supporting selected rebels fighting the group.
Russia and the Assad regime also say they are targeting Isis but class all opposition groups as “terrorists” and legitimate targets for their bombing raids.
In June, Russian planes hit a coalition-backed rebel group’s base with cluster munitions after allegedly ignoring attempts to warn them off.
Several members of the New Syrian Army (NSA) were killed in the attack on al-Tanf, where Russian officials allegedly did not respond to entreaties over the official communications channel.
The Kremlin’s jets reportedly retreated after the arrival of American planes, but returned after they left to carry out two devastating strikes during a supposed “cessation of hostilities”.
British special forces units were photographed near al-Tanf in June, as NSA fighters continued efforts to push Isis back from a strategic base and the borders with Iraq and Jordan.
Battles raged in Hasakah on Saturday, where residents said government air strikes had killed and injured dozens of civilians, cut off the electricity supply and caused food shortages.
The city and surrounding areas came under the control of militias led by the YPG in 2012 after regime forces withdrew from the predominantly Kurdish region to focus on fighting rebels elsewhere in Syria.
In a statement carried by Syrian state media, the general command of the armed forces accused Kurdish forces of “attacking state establishments, stealing oil and cotton, disrupting examinations, carrying out abductions, and causing a state of chaos and instability”.